1. Field of the Invention
The invention relates to a motor control apparatus for driving a brushless motor, and an electric power steering system provided with that kind of motor control apparatus.
2. Description of the Related Art
Electric power steering systems are known which apply steering assist force to a steering mechanism of a vehicle by driving an electric motor according to steering torque applied by a driver to the steering wheel. Conventional brush motors have come to be widely used for electric motors in electric power steering systems. However, from the viewpoint of improving reliability and durability, as well as reducing inertia and the like, brushless motors have also come to be used in recent years.
In order to control the torque generated by the motor, a motor control apparatus typically detects the current value supplied to the motor and performs PI control (i.e., proportional integral control) based on the difference between a target current value to be supplied to the motor and the detected current value. For example, a motor control apparatus that drives a three-phase brushless motor is provided with two or three current sensors to detect the current values of two or more phases.
However, a motor control apparatus in an electric power steering system requires current sensors that detect a large current of equal to or greater than 100 A. These kinds of current sensors are physically large, which makes it difficult to make the motor control apparatus small. Also, providing the current sensors increases the production cost of the motor control apparatus and also increases power consumption. One possible way to solve these problems is to eliminate the current sensors from the motor control apparatus and control the motor control apparatus by open loop control according to a circuit equation of the motor.
Incidentally, Japanese Patent Application Publication No. 2006-14474 (JP-A-2006-14474) describes an invention that estimates the value of power supply current supplied to a motor drive circuit based on the power supply voltage and the power consumed by a brushless DC motor. Japanese Patent Application Publication No. 2001-268980 (JP-A-2001-268980) describes a motor control apparatus that controls a motor in a feedback manner based on the difference between a current estimated value obtained by calculation from the battery voltage and the rotation speed of the motor and the like, and a set current target value. Japanese Patent Application Publication No. 2002-234457 (JP-A-2002-234457) describes a motor control apparatus that calculates the difference between the design characteristics of a motor and the actual control characteristics of the motor based on a current command value and a motor current value, and then feeds back that difference, Japanese Patent Application Publication No. 2005-229768 (JP-A-2005-229768) describes a brushless motor drive circuit which estimates the current value of current flowing between a power supply and a brushless motor based on the rotation angle of the brushless motor and the wiring resistance value and the like, and determines that there is an abnormality when the estimated current value deviates from a detection current value by a predetermined value or more.
However, in a motor control apparatus having no current sensors, the current value of current that is actually flowing to the brushless motor (hereinafter referred to as the “actual current value”) is unable to be detected. As a result, due to a voltage drop resulting from the actual current value deviating from the current command value, the brushless motor is unable to be accurately controlled.